1. Field of the Invention
The present invention relates to a DC-to-AC electric power converting apparatus for use in an AC power supply system such as an uninterruptive power supply system. More particularly, the present invention relates to an electric power converting apparatus of a high frequency intermediate link system in which high frequency electric power is transmitted/received via an insulating transformer.
2. Description of the Related Art
The structure of a conventional apparatus will be described with reference to FIG. 17. FIG. 17 is a block diagram of a conventional DC-to Ac power converting apparatus as disclosed in IEEE PESC '88 Record, pp658-663, 1988. Referring to the drawing, reference numeral 1 represents an inverter circuit, 2 represents a transformer the input of which is connected to the inverter circuit 1 and 3 represents a cyclo-converter circuit connected to the output of the transformer 2. Reference numeral 4 represents a filter circuit connected to the output of the cyclo-converter circuit 3 and 5 represents a current detector for detecting the output current from the cyclo-converter circuit 3. Reference numeral 6 represents a carrier signal generator, 7 represents a reference voltage signal generating circuit and 8 represents an absolute circuit. Reference numeral 9 represents a PWM circuit, 10 represents an inverter switching circuit and 11 represents a cyclo-converter switching circuit. The inverter circuit 1 comprises four semiconductor switching devices S1 to S4, while the cyclo-converter circuit 3 comprises four semiconductor switching devices S5, S6, S5A and S6A. The filter circuit 4 is an LC filter circuit comprising a reactor and a capacitor. Reference numerals 12 and 13 respectively represent a DC power source and a load circuit connected to the DC-to-AC electric power converting apparatus according to the present invention.
Then, the operation of the above-described conventional apparatus will be described with reference to FIG. 18. As shown in the uppermost portion of FIG. 18, reference voltage signal V* in the sine waveform transmitted from the reference voltage signal generating circuit 7 is converted into absolute signal .vertline.V*.vertline. by the absolute circuit 8. The absolute signal .vertline.V*.vertline. is, together with a carrier signal transmitted from the carrier signal generator 6, supplied to the PWM circuit 9. As a result, the PWM circuit 9 transmits two types binary signals Ta and Tb. That is, the binary signal Ta, the level of which is changed in synchronization with the timing at which the amplitude of the absolute signal .vertline.V*.vertline. and that of the carrier signal are allowed to coincide with each other, and the binary signal Tb, the level of which is changed in synchronization with the last transition of the carrier signal, are transmitted. Then, the binary signal Ta and Tb are supplied to the inverter switching circuit 10 so that ON/OFF signals T1 to T4 for switching on/off the four semiconductor switching devices S1 to S4 constituting the inverter circuit 1 are transmitted. That is, the ON/OFF signals T1 and T3 are the same as the binary signals Tb and Ta, respectively. The ON/OFF signals T2 and T4 are the signals obtained by respectively inverting the sign of the binary signals Tb and Ta. When the level of the ON/OFF signals T1 to T4 is high, the corresponding semiconductor switching devices S1 to S4 are switched on. When the same is low, the corresponding semiconductor switching devices S1 to S4 are switched off. As a result of the structure shown in FIG. 17, the relationships among the semiconductor switching devices S1 to S4 and the secondary voltage V2 of the transformer 2 are expressed as follows: EQU When the switches S1 and S3 are switched on: V2=0 EQU When the switches S1 and S4 are switched on: V2=Vdc EQU When the switches S2 and S3 are switched on: V2=-Vdc EQU When the switches S2 and S4 are switched on: V2=0 (1)
where symbol Vdc denotes the DC output voltage from the DC power source 12.
Therefore, when the semiconductor switching devices S1 to S4 constituting the inverter circuit 1 are switched on/off in response to the ON/OFF signals T1 to T4, V2; becomes AC voltage the pulse width of which has been modulated as shown in FIG. 18.
When the binary signal Tb, the reference voltage signal V* and output current icc from the cyclo-converter circuit 3 transmitted from the current detector 5 are supplied to the cyclo-converter switching circuit 11, ON/OFF signals T5, T6, T5A and T6A for respectively switching on/off the four semiconductor switching devices S5, S6, S5A and S6A constituting the cyclo converter circuit 3 are transmitted from the cyclo-converter switching circuit 11. It is assumed that the polarity of the output current icc is defined in such a manner that the direction, in which the output current icc is supplied to the load circuit 13, is positive. When the polarity of the icc is positive, the semiconductor switching device S5 or S6 is switched on/off. When the same is negative, S5A or S6A is switched on/off.
As a result of the structure arranged as shown in FIG. 17, the relationship between the output voltage Vcc from the cyclo-converter circuit 3 and the secondary voltage V2 of the transformer 2 is expressed as follows: EQU When S5 or S5A is switched on: Vcc=V2 EQU When S6 or S6A is switched on: Vcc=-V2 (2)
Therefore, when the ON/OFF signal T5 or T5A is made the same as the binary signal Tb and and when the ON/OFF signal T6 or T6A is made the signal formed by inverting the sign of the binary signal Tb, the polarity of Vcc becomes positive. When the ON/OFF signal T5 or T5a is made the signal formed by inverting the sign of the binary signal Tb and when the ON/OFF signal T6 or T6A is made the same as the binary signal Tb, the polarity of Vcc becomes negative. As a result, the cyclo-converter switching circuit 11 discriminates the polarity of the reference voltage signal V* and the output current icc from the cyclo-converter circuit 3 respectively supplied from the reference voltage signal generating circuit 7 and the current detector 5. Thus, the ON/OFF signals T5, T6, T5A and T6A as shown in FIG. 18 are generated from the binary signal Tb supplied from the PWM circuit 9 in accordance with the thus discriminated polarity. In accordance with this sine-wave voltage, the pulse width of which has been modulated and which is as shown in the lowermost portion of FIG. 18, can be obtained as the output voltage Vcc from the cyclo-converter 3. When the obtained output voltage Vcc is then supplied to the filter circuit 4, sine-wave voltage VL, from which the high frequency component has been eliminated due to the PWM operation, is supplied to the load circuit 13. When the frequency of the carrier signal is raised sufficiently with respect to the frequency of the reference voltage signal V* at this time, the load voltage VL to be supplied to the load circuit 13 becomes the voltage from which the high frequency component has been sufficiently removed due to the PWM operation and the amplitude and the phase thereof have been made substantially the same as those of the reference voltage signal V*. FIG. 18 illustrates a switching pattern when the load circuit 13 has been made the linear load of the delay power factor.
As described above, the conventional DC-to-AC electric power converting apparatus receives DC electric power and transmits AC electric power in accordance to the reference voltage signal. The above-described DC-to-AC electric power converting apparatus is usually called "a high frequency intermediate link type electric power converting apparatus" since the high frequency electric power is supplied/received via a transformer. Heretofore, structures employing high frequency intermediate link type electric power converting apparatus in an AC power source apparatus such as the uninterruptive power supply system have been realized to enable the size and the weight of the insulating transformer and the filter circuit to be reduced. However, the conventional DC-to-AC electric power converting apparatus has its structure arranged in such a manner that the PWM operation is performed in the inverter circuit 1. That is, when the conventional DC-to-AC electric power converting apparatus is desired to be made a multi-phase structure, both the inverter circuit 1 and the cyclo-converter circuit 3 must converted to multi-phase structures. Furthermore, it is necessary for both the inverter circuit 1 and the cyclo-converter circuit 3 to be controlled simultaneously in the form of a pair when the above-described conventional DC-to-AC electric power converting apparatus is desired to be used in the uninterruptive power supply system. As a result, flexibility in the constitution of the system, at the time of changing the capacity of the power source and employing the battery power supply system, is undesirably lost.